The invention relates to a gas friction pump having at least one outlet-side helical stage formed of an annular, helically extending gas delivery (gas pumping) channel.
Friction pumps encompass molecular and turbomolecular vacuum pumps. In molecular pumps a movable rotor wall and an immobile stator wall are so configured and so spaced from one another that the pulses imparted by the walls to the gas molecules situated between the walls have a predetermined, preferred direction. For this purpose, as a rule, the rotor and/or stator wall is provided with helically extending (thread or screw-like) depressions or ribs. Turbomolecular pumps have interengaging stator and rotor wheel series, similarly to a turbine; they need a pre-vacuum pressure of approximately 10.sup.-2 mbar. In contrast, molecular pumps deliver at pressures of 10 mbar and above so that the arrangement required for producing the pre-vacuum is much simpler.
Friction pumps of the above-outlined type, such as disclosed, for example, in German Offenlegungsschrift 3,705,912 are frequently used for evacuating vessels in which etching, coating or other vacuum treatments or manufacturing processes are performed. These processes involve the risk that solid particles may gain access to the pumps. In some processes such solid particles may come into being only during the compression of the gases, that is, during the passage of the pumped gas through the vacuum chamber. As an example there is mentioned the formation of aluminum chloride in case of aluminum etching or ammonium chloride in case of coating processes.
In case solid particles of the above-outlined type settle in the gas pumping channels of the vacuum pump, the diameter of the channels is reduced which results in a decrease of the output of the vacuum pump. Precisely in case of friction pumps which are, at least in the outlet-side zone, designed as molecular pumps, it has been found that undesired solid particles settle on the helical channel structure in the vicinity of the pump outlet.
It is a further risk that dust-like solid particles may gain access to the motor chamber which also accommodates bearings. Generally, these bearings are lubricated roller bearings which are exposed to an increased wear when dust is present.
In friction pumps which are utilized in the aboveoutlined pumping processes, an increased maintenance is necessary for the reasons stated. The removal of dirt from the gas pumping channels and the motor chamber necessitates a disassembly of the pump which is a complex operation, it causes a significant down time and therefore involves substantial expense.
Further, in helical pump stages, usually in the final pressure zone, reverse molecular and/or oil flows occur. These occurrences take place because there is practically no more molecular flow in the pumping direction and the optically free cross section of the helical channels is relatively large. Particularly in the final pressure operation gas circuits are generated in the helical stages. In the zone of the rotor wall the few, still-present molecules flow in the direction of gas pumping. In the zone of the bottom of the helical turns these gases flow in a reverse direction and cause a reverse oil flow. Therefore the risks are substantial that the oil molecules originating from the pre-vacuum pump gain access to the recipient vessel and thus adversely affect the process performed therein. Particularly in the manufacture of semiconductor components even the smallest amounts of oil vapor concentrations may prove to be extremely harmful.